Generally, equipment for optically broadcasting P programs to N subscribers performs two functions: one of which is a distribution function which consists in transmitting each of the programs N times for N subscribers; and the other of which is a selection function which consists in transmitting over each of N subscriber links only that one of the programs which is desired by the subscriber concerned, selected from the P programs made available. In one such broadcast equipment, the function of distributing the P programs to the N subscribers may be performed by wave guides, such as optical fibers or guides which are integrated into a medium, or they may be provided by optical systems such as lenses. The selection function for selecting one out of P programs by each subscriber may be provided in particular by photodiodes.
The article entitled "A high-speed opto-electronic matrix using heterojunction switching photodiodes" by H. Hara et al, published in the IEEE Journal of Quantum Electronics, vol. QE-17 No. 8, August 1981, pages 1539 to 1545 describes the principle of providing an opto-electronic switching matrix based on photodiodes with reference to FIG. 1 provided therein. In this example, the individual power supply to each photodiode is used to provide switching, said power supply takes place via a controlled switch associated with the photodiode for obtaining the function of selecting one inlet signal from a plurality at the outlet from the matrix. Thus, when one of the photodiodes is connected to the positive power supply terminal, this photodiode converts the received optical signal or received photons into an electrical current which is detected and transmitted over the outlet which is common to a column of photodiodes. However, when one of the photodiodes is connected to the negative power supply terminal, this photodiode is DC biased and has an electric current passing therethrough of about 10 mA, thus preventing any detection of a received optical signal. The passage of such a current is necessary in order to cancel the photovoltaic effect of the photodiode.
The article entitled "A multiservice single fiber subscriber network with wide band electro-optical switching" by P. B. Hesdahl et al appearing in ECOC 83--9th European Conference on Optical Communication and published by Elsevier Science Publishers P.V. (North Holland), 1983 page 331 to 334 also describes the principle of making an opto-electronic selection matrix from photodiodes each of which is followed by an associated preamplifier. In this matrix, individual switching control of the photodiodes is applied both to the power supply to each photodiode and to the power supply to the associated preamplifier. This configuration, as in the preceding matrix configuration, uses discrete components and provides point decoding of signals transmitted to intersection points; in addition, relative to the preceding matrix, this matrix provides improved isolation between photodiodes and avoids the components consuming electricity when the desired received signal is not detected.
An opto-electronic selector having photodiodes associated with switching transistors for addressing them in matrix form is also known from French Pat. Nos. 2441309 and 2445678, in particular. In such an arrangement, the photodiodes are identified by the intersections of lines and columns. The photodiodes individually receive the inlet optical signals to the selector, and their electrical signal outlets are connected in common to the outlet from the selector. All of the photodiodes are biased from a common voltage source. Bias is applied to each photodiode under the control of two MOS transistors connected in series. One of these transistors is attributed to all the photodiodes of a given line and is switched on by applying a voltage to its gate in response to a command selecting the line corresponding to the photodiode in question. The other transistor is attributed to a single photodiode and is switched on by a voltage being applied to its gate under the control of a column selection suitable for said photodiode, said column selection control voltage being applied in common to the gates of all of the transistors attributed to the photodiodes in said column. When the photodetector selected in this manner receives a light signal, current is transmitted to its electrical signal outlet. In such a selector, it is essentiaal that the non-selected photodiodes are unable to dispose of the charges which are created when they receive an optical signal. This implies that the transistors attributed to the unaffected columns and also those attributed to the unaffected lines should be effectively turned off, i.e. they should have zero or very low leakage current in order not to transmit interference currents from the non-selected photodiodes. As a consequence, although this solution is of interest for reducing the number of control links by using a matrix addressing system for the photodiodes, it nevertheless remains difficult to implement in practice, in particular in face of the difficulty in obtaining high-quality photodiodes (sensitivity of about 0.4 A/W and operating capacitance of less than 0.3 pF) on the same substrate as MOS transistors having an on/off resistance ratio of 10.sup.7.
The aim of the present invention is thus to provide an opto-electronic selector of the type in which the photodetectors are matrix addressed, but further presenting increased insensitivity to noise and better isolation between channels than the above-mentioned known structure of this type, and also being readily suitable for integration.